This invention relates to the apparatus and methods suitable for cryogenic pump systems, either mobile or stationary, incorporating a medium or high pressure reciprocating cryogenic pump, operating under difficult conditions where little if any Net Positive Suction Head (NPSH) is available. Examples of such especially difficult pumping applications are small and/or mobile systems and situations where providing NPSH by pressure building of the cryogen storage so as to provide NPSH is not desired. Also difficult are systems pumping to medium (about 1,500 psig) or high (about 3,000 psig or higher) pressures where the heat of compression is great. Start-stop service against medium or high pressures increases the difficulty, if cryogen venting and/or pump priming is not desired, the usual case.
One example incorporating these problems are vehicles using a liquefied low temperature (cryogenic) fuel such as LNG (liquefied natural gas), methane, hydrogen or the like used in on-board cryogenic storage and pumping systems for providing medium or high pressure cryogen vapor for measured, direct injection to the vehicle's engine so as to provide motive power fuel to the vehicle. Unlike normal diesel fuel or gasoline, the desired precise metering and timing injections of cryogen fuel in the liquid form into the engine has not been feasible, therefore injection of pressurized vapor is desirable. Using LNG as an example, the high pressure cryogen vapor would be referred to as compressed natural gas (CNG). Accordingly, the engine of the vehicle on which the system is located may receive it's CNG fuel from the LNG source, but after being pumped to a desired pressure and then vaporized.
Another example incorporating these problems are very small cryogenic liquid systems where it is desired to fill steel high pressure (about 2,500 psig) cylinders with warmed cryogen vapor, using cryogenic liquid stored in small liquid cylinders as the source of the vapor. There are many other examples where very low NPSH reciprocating cryogenic pumps would be of great utility.
While a number of cryogenic pumps have been developed for low NPSH service; the difficulties of providing pumps suitable for an on-board vehicle medium or high pressure cryogenic pump, or for other small pumping systems have proven too great. Onboard cryogenic pumping systems intended to operate while the vehicle is in motion are especially difficult, as the motion of the vehicle (and consequently the cryogen storage tank), tends to make the stored cryogen come to and remain at equilibrium conditions, i.e. boiling from any heat incursion through the tank's insulation. Included in previous low NPSH designs are U.S. Pat. No. 3,011,450 issued Dec. 5, 1961; No. 3,023,710 issued Mar. 6, 1962; No. 3,263,622 issued Aug. 2, 1966; and No. 3,277,797 issued Oct. 11, 1966 --all to the present inventor. In particular the '710, the '622 and '797 patents show pumps where the intake valve into the pumping chamber is caused to open by the mechanical action of the piston rod retracting from a center opening in a hollow piston, a type of action commonly referred to as a "lost motion" action as the piston does not move as far as does the piston rod. This mechanically opening of the intake valve reduces one principal need for NPSH, that of causing the intake valve to open by a reduction in pressure. Another more recent U.S. Pat. No. 5,188,519 issued Feb. 23, 1993 to I. S. Spulgis shows a similar "lost motion" action in opening the intake valve mechanically. However, no cryogenic pump has been totally satisfactory in its operation when handling cryogenic liquids at the very low NPSH encountered in small and/or mobile applications. Attention is directed at the explanation of NPSH contained on page 27of MVE Data Book P/N 10517513 8/96 making the statement that no pump can operate on saturated liquid (zero NPSH) cryogen, a widely held view.
It has long been recognized that LNG and the like could be a most useful fuel for buses, trucks and other transportation systems requiring reciprocating Diesel engines or other types of internal combustion engines. However, the clean burning and efficient Diesel cycle requires substantial injection pressures of its fuel for best overall performance. It is also desired to carry the fuel in the cryogenic liquid state, so as to save the weight and space of high pressure gas cylinders. The desired solution of carrying on-board the LNG as a low pressure liquid, but providing high pressure gas to the engine, is one requiring an on-board cryogenic pump. As stated earlier, the performance of this pump must be quite unusual as the LNG in the storage tank will tend to stay at equilibrium conditions (due to the vibration of the vehicle), thus providing near zero Net Positive Suction Head (NPSH) to the pump's inlet, a condition under which most known medium or high pressure cryogenic pumps cannot reliably operate, especially as the tank becomes nearly empty. Furthermore, for many reasons, it is not desirable to vent to the atmosphere natural gas vapor from the on-board storage and pressurizing system; accordingly the traditional methods/techniques utilized in the cryogenic pump industry to provide prime or NPSH are not appropriate.
The attractiveness of utilizing the cold potential represented by LNG vaporization for some other useful purpose has been long recognized; i.e. U.S. Pat. No. 3,027,727 issued Apr. 3, 1962 to G. F. Farmer and No. 3,363,425 issued Jan. 16, 1968 to R. H. Williamson and many others; but no system has functioned well enough to be accepted and widely used. At the January 1988 Energy Sources Technology Conference and Exhibition, and reported in ASME Paper 88-ICE-21 entitled "LNG (Liquefied Natural Gas) as a Fuel and Refrigerant for Diesel Powered Shrimp Boats", where the vehicle is a boat. U.S. Pat. Nos. 3,685,310 issued Aug. 22, 1972 and 3,740,961 issued Jun. 26, 1973 to H. Fischer describes an ammonia system (characterized as an "open cycle") where the liquid ammonia provides both the cooling for in-transit refrigeration and the resultant gaseous ammonia can be fed to the engine to augment the engine's fuel supply. U.S. Pat. No. 3,823,568 issued Jul. 16, 1974 to T. Bijasiewicz et al describes a system wherein land and water vehicles use a cryogenic fuel which can also cool the passenger compartment. U.S. Pat. No. 5,211,029 issued May 18, 1993 to R. Uselton et al describes a so called "negative energy storage system", for storing a cooling effect for later use. U.S. Pat. No. 5,277,038 issued Jan. 11, 1994 to P. Carr describes a thermal storage system for storing cooling (and/or heating) for later use in cooling (and/or heating) the passenger space of the vehicle in which it is located.
The use of slush carbon dioxide (a mixture of liquid and solid) to store for later use a refrigeration effect has been envisioned in U.S. Pat. No. 4,100,759 issued Jul. 18, 1978; in No. 4,211,085 issued Jul. 8, 1990; in No. 4,224,801 issued Sep. 30, 1980; in No. 4,693,737 issued Sep. 15, 1987; in No. 4,695,302 issued Sep. 22, 1987; and in No. 4,995,234 issued Feb. 26, 1991--all either to the present inventor or with the present inventor as a named inventor. The '234 patent uses the vaporization of LNG to create a useful refrigerating effect, which is stored in the slush mixture by effecting a phase change of some of the liquid carbon dioxide to a solid, for later use.
Even more recently, the U.S. Dept. of Energy (DOE) in a Small Business Innovation Research 1996 Program Solicitation, No. DOE/ER-0686 identified "Liquid Natural Gas Storage for Heavy Vehicles" as a technical topic in which the DOE has a R & D mission. In this Solicitation, on-board medium pressure (approx. 500 psig) and high pressure (approx. 3,000 psig) cryogenic pumps were identified as areas where innovation was specifically desired, as an existing component or practice that requires improvement.
However, and despite these publicized needs, the difficulties inherent to an on-board LNG storage, pumping and vaporizing system/station suitable for medium or high pressure injection of the natural gas into a Diesel engine have prevented such a system from being widely used. Also while many have proposed systems for utilizing the refrigerating effect obtained from vaporizing a cryogenic fuel such as LNG, none also have reached the stage where commercial success resulted. The definition of cryogenic liquid as used herein is a gas whose critical temperature is below terrestrial temperatures; i.e. below about -85.degree. F. Examples are (in the liquid state) nitrogen, oxygen, argon, methane, hydrogen and natural gas (LNG). Where the term LNG is used herein as an example, it equally applies to any liquefied fuel stored at cryogenic temperatures, such as, but not limited to, methane or hydrogen.